Method of making a reinforcement preform and a reinforced composite therefrom

ABSTRACT

A preform for impregnation of a metal or ceramic matrix material comprising a multiplicity of reinforcement particles bonded together so as to define a three-dimensional, open-cell reticulum comprising a plurality of randomly oriented thread-like portions interconnected one to the next via a plurality of nodes. A reinforced composite made from such preform and method of making the preform is disclosed/claimed wherein the reinforcement particles are mixed with prepolymers used to produce a fugitive binder for the particles comprising a foamed polymer, and the particles align themselves with the polymer portions of the resulting foam.

This is a division of application Ser. No. 08/169,251 filed on 20 Dec.1993, now U.S. Pat. No. 5,427,853.

This invention relates to reinforced composites including a uniquepreform adapted for impregnation with a matrix material and a method ofmaking same.

BACKGROUND OF THE INVENTION

It is well known in the art to reinforce ceramics and light metals suchas Al, Mg, etc. (i.e., the matrix material) by dispersing a variety ofreinforcement particles throughout the material. Common reinforcingparticles are carbon/graphite, Al₂ O₃, glass, mica, SiC, wollastonite,alumino-silicate (e.g., Kao-wool), inter alia. Typically, thereinforcing particles will comprise about 3% by volume to about 30% byvolume of the composite. The particles may be essentially equiaxed, orelongated (e.g., whiskers and fibers), and serve to improve themechanical properties (e.g., strength, toughness, friction, fatigueresistance and wear resistance) of the composite over the properties ofthe metal or ceramic matrix material alone. Popular elongated particles(hereafter, fibrils) typically have an aspect ratio (i.e., lengthdivided by diameter) of between about 3 to about 20. Their lengths varyfrom about 50 to about 200 microns and have diameters less than about 10microns. Preferable lengths are between about 75 microns and 100microns.

Reinforced composite materials are typically made by either one of twobasic processes. In one process, reinforcements are simply mixed withthe matrix material (e.g., molten metal) and together therewith cast asa slurry into an appropriate mold for shaping the finished product. Inthe other process, a self-supporting preform is made in the desired sizeand shape from the reinforcements, and the preform subsequentlyimpregnated with the metal or ceramic matrix material by well knownpressure or vapor infiltration techniques. In the latter process, it isparticularly desirable that the preform be made to the actual size andconfiguration that it will be used in the finished molding/casting sothat little or no subsequent machining or shaping thereof is required.According to this latter process, the preform is formed in anappropriate mold to the desired size and shape which may conform either(1) to that of the finished composite article, or (2) to only aparticular defined portion of the finished composite article (e.g., areinforced portion of an otherwise reinforcement-free article).Thereafter the preform is impregnated with the desired matrix material.The preform may be formed in a first mold and then transferred to asecond mold where it is impregnated with the matrix material, or thepreform may be formed and impregnated in the same mold.

A known technique for forming the preform comprises mixing thereinforcing particles uniformly throughout a fugitive binder (e.g., wax,polystrene, polyethylene, etc.), injecting the binder-particle mixtureinto a mold, removing (e.g., volatizing or dissolving) the binder, andfinally bonding the residual particles together into a self-supportingstructure. As is well known in the art, particle bonding may be achievedby sintering, or by providing the particles with a coating of colloidalSiO₂ which, upon heating, acts like a high temperature glue for holdingthe particles together. Some of the disadvantages of the mix and moldtechnique are (1) the ofttimes inability to completely fill the moldcavity with a homogeneous mixture of the particles, (2) upper limits onthe amount of particles that can be used while still being able toinject the mix, (3) the need to remove a large volume of binder (i.e.,about 60% to about 85% by volume of the preform mixture) and (4)difficulty in avoiding planar, unreinforced areas which arise from flowlines and mating lines in mold.

All in all, the use of preforms is considered to be the preferred way tomake composite materials. However, it has heretofore been difficult touniformly and completely impregnate the preforms at commerciallyacceptable rates. It would be desirable to provide a self-supportingpreform, which is readily impregnated without untoward sacrifice of thephysical property(s) sought to be enhanced by the reinforcements.

It is an object of the present invention to provide a self-supporting,heterogeneous, reinforcement preform, wherein reinforcement particlesare in the form of an open-cell reticulum, defining a plurality ofreinforcement-free pores/cells which are readily fillable with matrixmaterial. It is a further object of the present invention to form theaforesaid preform while foaming a fugitive binder therefor. It is astill further object of the present invention to provide aheterogeneous, reinforced material comprising a metal or ceramic matrixphase embedding a reinforcement phase which reinforcement phasecomprises a three-dimensional reticulum comprising a plurality ofrandomly oriented thread-like portions interconnected one to the nextvia a plurality of nodes. These and other objects and advantages of thepresent invention will become more readily apparent from the descriptionthereof which follows.

The present invention permits the making of readily impregnated metalmatrix composites (hereafter MMCs) which have good wear-resistance andfrictional properties, and readily impregnated ceramic matrix composites(CMCs) which are tougher than ceramics made without the reinforcements.The present invention provides a heterogeneous composite having asubstantially consistent distribution of the reinforcements throughoutthe composite but with concentrations of the reinforcements at certainlocations throughout and very little reinforcements elsewhere in thecomposite.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided aself-supporting preform adapted to be embedded in a metal or ceramicmatrix material and comprising a multiplicity of discrete reinforcingparticles bonded together in the form of a three-dimensional, open-cellreticulum. The reticulum comprises a plurality of randomly oriented,thread-like portions interconnected one to the next via a plurality ofnodes, and together therewith defining a plurality of interconnected,interstitial pores/cells therebetween. The pores/cells will preferablyhave diameters between about 100 and about 200 microns. In accordancewith one preferred embodiment of the invention, the particles compriseceramics such as Al₂ O₃, Al₂ O₃ ·SiO₂ (4% SiO₂), SiC, CaSiO₃, BN, Si₃N₄, Al₂ O₃ ·SiO₂ (53% SiO₂) and K₂ O·6TiO₂. In accordance with anotherpreferred embodiment, the particles will be fibrils each having anaspect ratio of between about 3 and about 20. The preform is filled withan appropriate matrix material (i.e., metal or ceramic) so as to providea heterogeneous, reinforced composite material comprising the matrixphase embedding the reinforcement phase wherein the reinforcement phasehas the structure and configuration of the aforesaid reticulum.

In accordance with another aspect of the present invention, there isprovided a method of making such a reticulated preform for reinforcing acomposite comprising the steps of: (1) providing first and secondprepolymers adapted to react with each other in such a manner as to foamand form a three-dimensional, fugitive, polymer reticulum comprising aplurality of randomly-oriented, thread-like portions interconnected oneto the next via a plurality of nodes which together define a pluralityof interconnected, interstitial cells; (2) mixing a plurality ofdiscrete reinforcing particles with one or both of the prepolymers; (3)reacting the prepolymers (i.e., with the particles present) together ina mold having a mold cavity conforming substantially to the desiredshape of the preform, and so as to foam the reacting mixture as itpolymerizes, and so as to concentrate the particles at the polymerportions of the foam which define the pores/cells; (4) removing thepolymer (e.g., as by burning or dissolution) so as to leave theparticles in place; and (5) bonding the particles together to provide asubstantially self-supporting preform adapted to be filled with a matrixmaterial. Bonding may be effected by sintering or by means of an SiO₂binder as is well known in the art. Thereafter the preform isimpregnated with the desired matrix material (e.g., metal or ceramic).

Any foamable polymer can be used provided it yields an open-cell foamand can be readily removed (e.g., dissolved or burned-out) afterpolymerization is complete. Polyurethane and silicone foams are seen tobe particularly convenient for this purpose. Regardless of theparticular polymer system chosen, the reinforcing particles (preferablyfibrils) are mixed with one or both of the prepolymers used to form thepolymer, and then the prepolymers mixed together.Dispersants/surfactants such as polymeric fatty ester, polyoxyethylenealcohol, ethoxylated methyl carbitol oleate, or ethoxylated alcohol maybe added to the prepolymers along with the reinforcements, to facilitatemixing and dispersion of the reinforcements in the prepolymer(s). In thecase of the polyurethane foams (which are made by reacting a polyol witha polyisocyanate), the reinforcement particles may be mixed either withthe polyisocyanate prepolymer or the polyol prepolymer. Preferably, theparticles will be added to the polyol which has a lower viscosity andhence permits a more ready mixing of the particles therewith. In thecase of the silicone foams (which are made by a condensation reactionbetween silane and silanol-containing compounds), the particles may bemixed with either or both of the prepolymers, but preferably with thesilanol for the same reasons as set forth above for the polyol. Ineither case during their reaction, bubbles are formed (i.e., H₂ in thesilicone reaction, and CO₂ in the urethane reaction) which serve to foamthe reactant during the polymerization reaction. The reactants caneither generate their own bubbles or foaming agents can be added to thereactants to form the bubbles. The bubbles in the polymerizing mixturecause the polymer to foam and expand so as to fill the mold with anopen-cell, sponge-like solid. The bubbles formed during the reaction ofthe prepolymers cause rearrangement of the fibrils along with thereacting prepolymers to sites bordering the bubbles. More specifically,during foaming a thin web of the reacting materials initially formsbetween each bubble. Eventually the web ruptures causing the materialcomprising the web to retract or consolidate into a plurality ofrandomly-oriented, thread-like structures interconnected one to the nextvia a plurality of nodes all together defining a plurality ofinterconnected, interstitial pores or cells. The reinforcement particlesremain with the reactants (and the polymer formed therefrom) andaccordingly retract with the reactants. Hence when the web ruptures andretracts, reinforcements become concentrated at the thread-like portionsand nodes forming the reticulum to the exclusion of the pores/cells.Subsequent removal of the polymer leaves a three-dimensional reticulumwherein the reinforcing particles are concentrated at the wall segmentsof the solid foam, and, in the case of fibrils, have their major axesaligned with each other in the thread-like portions of the reticulum.More specifically, following completion of the polymerization reaction,the polymer-particle reticulum is preferably heated in air sufficientlyto burn off the polymer (i.e., to about 1000° C. to remove urethane) andleave the particles in place. Thereafter, the residual particles arefurther heated to bond them together sufficiently to form aself-supporting preform. In one embodiment using Al₂ O₃ fibrilreinforcements, the particles are heated to at least about 1300° C. andpreferably about 1500° C. to sinter the particles together. In anotherembodiment, the particles are coated with either colloidal silica orsilica gel, prior to being mixed with the prepolymer. During heating,the SiO₂ precursor wicks to the nodes where the particles contact eachother, bonds to adjacent similarly coated particles as well as promotesbonding of the aluminum matrix material to the particles. When siliconefoams are used, Si-- O--Si bonds are formed during the foaming reaction,and are comparable to SiO₂. Hence when silicone foams are used, thereinforcements do not require a separate addition of SiO₂ to the systemto facilitate interparticle binding.

After the particles have been heated and bonded together, the preform iscooled and transferred to the mold/die used to form the finished productand therein impregnated/infiltrated with the desired matrix material.Pressure-casting (e.g., die casting or squeeze casting) is the preferredtechnique for impregnating the preform with metals such as aluminum ormagnesium to make MMCs. In the case of CMCs, on the other hand, chemicalvapor or liquid slurry infiltration techniques may be used as arewell-known in the art. For example, in the case of chemical vaporimpregnation, the preform may be exposed to flowingmethyltrichlorosilane in a hydrogen gas carrier. At elevatedtemperatures, the gas decomposes on the preform surface essentiallyaccording to the reaction SiCl₃ CH₃ (g)═SiC+3HCl(g). This reaction canbe carried out at about 981° C. and 6.7 kPa total gas pressure, using agas comprising 16 percent SiCl₃ CH₃ in H₂ flowing at a rate of about 1.7liters/minute.

Ceramic reinforced composites made in accordance with the presentinvention provide wear-resistant MMCs and tough, fracture-resistantCMCs. In the case of the MMCs, the ceramic reinforcement is much harderthan the light matrix metal (e.g., Al, Mg) embedding it, and accordinglyprovides wear resistance to the softer material. In the case of CMCs,the reinforcing particles serve to intercept and prevent the propagationof continuous cracks throughout the material that would otherwise formin, and cause the destruction of, brittle ceramics. For bothapplications, it is desirable that there be a high frequency of theceramic reinforcement in any cross section of the material. Hence, it isdesirable that the preform have an open-cell structure in which thecells are quite small. Accordingly, while it is possible to makereinforced composites according to the present invention in which thereticulum has pore sizes varying from about 30 to about 1500 microns, itis preferred that the pores/cells be less than about 150 microns indiameter. On the other hand, if the pore sizes are too small (i.e., lessthan about 50 microns in diameter), it becomes increasingly moredifficult to impregnate it with molten metal at an acceptable rate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will better be understood when considered in the light ofthe following Figures in which:

FIG. 1 is a draftsman's illustration of the structure of athree-dimensional, open-cell, reticulum in accordance with the presentinvention;

FIG. 2 is a scanning electron micrograph of a preform made in accordancewith a preferred embodiment of the present invention (i.e., afterremoval of the polymer);

FIG. 3 is a higher magnification scanning electron micrograph of thepreform of FIG. 2; and

FIG. 4 is a back-scattered electron image of a cross-section of aceramic preform made in accordance with the present invention, which hasbeen impregnated with aluminum alloy.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a draftsman's illustration of an open-cell reticulum of thetype formed by the process of the present invention. The reticulum 2comprises a plurality of thread-like portions 4 randomly orientedthroughout the reticulum 2 and joined to adjacent thread-like portions 4via a plurality of nodes 6 all defining a plurality of interconnected,interstitial pores/cells 8. As best shown in FIGS. 2 and 3, reinforcingparticles 10 concentrate themselves at the thread-like portions 4 andnodes 6 of the reticulum 2 to the substantial exclusion of theinterstitial pores/cells 8. Moreover, as best shown in FIG. 3, fibriloseparticles tend to align themselves lengthwise along the thread-likeportions 4 of the reticulum 2 and more randomly at the nodes as bestshown in FIG. 3.

A number of samples were made using polyurethane foam as thereticulum-forming polymer, fibrilose Al₂ O₃ as the reinforcingparticles, particle loadings varying from 0 to 18.1 percent by weight ofthe combined polyurethane-particle mix, and using essentially known RRIM(i.e., reinforced reaction injection molding) techniques followed bypolymer removal (i.e., burn-out) and fibril bonding steps to form aself-supporting preform. The fibrilose Al₂ O₃ used was commerciallyavailable from ICI America's Inc. under the trade name "Saffil" whichcontains Al₂ O₃ ·SiO₂ (4% SiO₂) fibrils. Saffil fibers initially vary inlength from about 13000 microns to about 51000 microns and are 3.0microns in diameter. These fibers are chopped into fibrils havinglengths of about 75 microns to about 100 microns. Preforms madetherefrom contained up to 3.8 volume percent Saffil fibrils. Inconnection with the fabrication of such samples, it was observedgenerally that, (1) when foaming was allowed to occur unconstrained, thedegree of expansion decreased with increasing fibril content, (2) thatmoist fibrils caused greater expansion than dry fibrils, and (3) thatunconstrained foams shrunk considerably when the polymer was burned out.When foaming took place in a closed reaction chamber, akin to a moldcavity, the resultant preforms (1) had a much finer cell size than theunconstrained foams, (2) did not shrink appreciably upon burn-out, (3)contained 3.1 volume percent fibrils (i.e., after polymer burn-out), and(4) were readily impregnable with aluminum alloy without crushing orcollapsing of their reticulated structure.

More specifically, the several samples were made using a polyurethanefoam making system marketed by the ICI Polyurethanes Group of ICIAmerica's Inc. under the trade designation XRS-8221. This particularpolyurethane system has an expansion ratio of about 10 to 1 (i.e., thevolume of the foam is about 10 times the volume of the prepolymerreactants), and yields open-cell foams. The XRS-8221 system'sformulation involves two prepolymer components, i.e., RIM 8900A andXRS-8901B. RIM 8900A contains about 65 percent by weight4,4'-diisocyanato diphenylenemethane and about 35 percent oligomershaving a similar structure. XRS-8901B is a blend of polyols andglycerine. Such reactants are typical of other polyurethane foam systemsand are generally referred to in the art as the "diisocyanate" and"polyol" reactants respectively.

The XRS-8221 material was first foamed without the addition of anyreinforcements. The amount of expansion was calculated from the "zeroporosity" density of the polyurethane which was reported by the supplierto be 1.22 grams/cc, and the measured bulk density of the finished foam(i.e., 0.09 grams/cc). Using this approach, the calculated expansion was1100 percent. The foam appeared to be essentially open-cell thoughperhaps some closed cells were still present. The foam was uniform andhad interstitial cell sizes ranging from about 100 to about 1000 micronsin diameter. Thermogravimetry (hereinafter TGA) was performed on thefoam in flowing air to determine the affects of temperature thereon. Itwas observed that weight loss occurred primarily in two stages. Thefirst being between 300°-420° C., and the second being between 450°-650°C. The remaining 10 percent of the urethane disappeared by the time1000° C. was reached. The same weight loss pattern was observed for allthe other samples tested including those containing the Al₂ O₃ fibrils.Accordingly it was concluded that heating to 1000° in flowing air wassufficient to completely remove the polyurethane.

In the remaining samples, Saffil fibrils were used as the reinforcingphase and were mixed in a food-type blender with either or both of theprepolymers depending on the particular sample. The fibrils were driedfor eight hours at 950° C. and stored in a desiccator prior to mixingwith the prepolymer. The diisocyanate and polyol prepolymers weretemperature equilibrated in water baths at 25° C. and stirred separatelyprior to mixing. The stirring was found to be particularly advantageouswith the "polyol" reactant because of the tendency for the glycerine toseparate out. 1.05 parts by weight diisocyanate was mixed with one partby weight polyol. Some samples were reacted in an open cup to allowunconstrained foaming whereas others were foamed in a capped cup tosimulate a closed system comparable to a closed mold/die cavity forconfining the reactants during foaming. After foaming, the urethane wasburned out and the remaining particles heated to bond them together.Following burn-out and bonding of the fibrils, the Al₂ O₃ reticulumremaining was characterized in a scanning electron microscope. All inall six foams were prepared using varying amounts of Saffil fibrils.Five of the samples were allowed to expand freely while the sixth samplewas contained/confined (i.e., in a capped cup) during reaction to limitits expansion and to simulate the conditions that would occur in aclosed mold/die.

EXAMPLE A

A fibril reinforced sample was prepared by mixing 52.0 grams ofdiisocyanate with 47.4 grams of polyol. Saffil fibrils were first addedto the diisocyanate in an amount equal to 12 percent by weight of thediisocyanate (i.e., 4 percent by volume of the mixture) which yielded avery gelatinous material. When mixed with the polyol, the fibril contentof the mixture was 6.8 percent by weight. Upon mixing with the polyol,foaming occurred and the mix expanded to about 1100 percent of theunexpanded volume of the constituents. The resulting foam was athree-dimensional, open-cell reticulum characterized by some voids, aswell as larger pores and a wider cell size distribution than wasobserved in the polyurethane foam made without the fibrils present. TGAwas performed on separate pieces of the foam taken at three differentlocations in the foam mass. Despite significant variations in porositythroughout the foam, the fibril loading remained relatively uniform withthe measured residual weights of the different pieces being 7.1 percent,7.1 percent and 6.4 percent which is good agreement among the samplesand generally consistent with the initial fibril content of 6.8 percent.The foam's bulk density was about 0.1 grams/cc. Using this value and thesupplier reported density of 1.22 grams/cc for polyurethane and 3.3grams/cc for the Saffil, the fibril content was calculated to be 0.2percent by volume before the polyurethane was removed. The sample washeated in air to 1000° C. to remove the polyurethane, which resulted inshrinkage of the foam by approximately 70 percent. Little, if any, shapechange (distortion) occurred as result of the shrinkage. Considering theshrinkage, the estimated volume occupied by the fibrils in the preformafter burn-out was about 0.7 percent and resulted in a rather fragilepreform.

EXAMPLE B

This sample was made by reacting 49.0 grams of diisocyanate with 48.3grams of polyol, but with the Saffil fibrils premixed with the polyolprior to the reaction. A much higher loading of Saffil fibrils was usedand resulted in a foam which was a three-dimensional, open-cellreticulum containing 18.1 percent by weight fibers and having a densityof 0.37 grams/cc. The foaming reaction was slower than the earliersamples, and the surface of the foam remained tacky for about 15 minutesafter the reaction should have been complete. The foam expanded onlyabout 175 percent, had a variable porosity and yielded a preform havinga fibril content of 3.8 percent by volume. The TGA behavior of this foamwas identical to that of the previous foams except that the retainedweight after removal of the polyurethane was 18.5 percent which was inexcellent agreement with the original loading of 18.1 weight percent.The foam shrunk only 50 percent during removal of the polyurethane. FIG.2 is a SEM micrograph of the Example B preform following burning out ofthe polyurethane, and clearly shows the sponge-like, open-cell,three-dimensional reticulum of fibrils which remains after the removalof the polyurethane. FIG. 3 is the same reticulum as shown in FIG. 2 butat a higher magnification. FIG. 3 shows the alignment of the severalfibrils 10 with each other and the concentration thereof along the axesof the thread-like portions 4 extending between the nodes 6 of thereticulum 2. This alignment of the major axes of the fibrils 10 with thelong axes of the thread-like portions 4 of the reticulum 2 was indeedsurprising since prior to foaming the fibrils 10 were randomly orientedthroughout the polyol. Hence, alignment of the fibrils 10 with thethread-like portions 4 and each other occurred during the foamingprocess by the action of the bubbles acting on the fibrils 10. Asclearly shown from the SEM, the fibrils 10 are concentrated at thereticulum structure itself leaving large pores/cells 8 therebetweenwhich are substantially free of any reinforcements. Hence while theoverall fibril loading of the foam is about 3.8 percent by volume, thefibril loading/concentration in the reticulum itself is significantlyhigher.

EXAMPLE C

In another sample, 44 grams of diisocyanate were mixed with 43 grams ofpolyol containing 6.9 grams of Saffil fibrils which upon foaming yieldeda three-dimensional, open-cell reticulum comprising 8.2 weight percentfibers (i.e., 0.7 percent by volume). The constituents expanded by about1200 percent during the foaming reaction and yielded a foam having adensity of 0.09 grams/cc. While the resulting foam had a more uniformpore distribution than sample A, the cell size and size distribution wasstill larger than had been obtained with the fibril-free foam. The TGAbehavior of this foam was similar to that observed for the other foamstested.

EXAMPLE D

Still another sample was prepared from 49 grams diisocyanate and 48.3grams of polyol containing 21.5 grams of wet Saffil (i.e., 0.5 percentmoisture). The 21.5 grams of Saffil corresponded to 18.1 weight percentof the fibrils in the finished foam. This test was performed todetermine the effects of water since water can react directly withdiisocyanate to produce CO₂. The fibrils were moistened by enclosingthem for three days in a desiccator which contained a beaker of water.The TGA analysis of the resulting fibrils showed that water absorptionwas approximately 0.5 percent of the fibril weight. Subsequentprocessing was identical to that described in Example B. The foamproduced by this test was a three-dimensional reticulum which expandedover twice as much as the foam produced in Example B, but only abouthalf that produced in Examples A and C. Upon removal of thepolyurethane, the foam yielded a preform containing about 2.0 percent byvolume fibrils and a pore size similar to that found in Example B.

EXAMPLE E

Finally, a sample was prepared from 75.0 grams of diisocyanate and 71.0grams of polyol containing 32.3 grams of Saffil fibrils which produced afinished foam which was a three-dimensional, open-cell reticulumcontaining 18.1 weight percent fibrils. These reactants were placed in aclosed container having a volume of about 250 cc so as to confine theexpansion to that volume. This volume was about 25 percent greater thanvolume of the prepolymer/fibril mixture. Thus, expansion was limited toabout 25%, which is considerably less than was observed with the opencontainer. Following the reaction, the foam was left in the containerfor 18 hours and then removed by cutting away the container from thefoam. The foam had a density of 0.63 grams/liter, a fine uniformporosity, average pore sizes of about 100 microns, and yielded a preformcontaining 3.1 volume percent of the fibrils. The resulting foam wassofter than the foams which were allowed to freely expand. During TGAremoval of the polyurethane, some sagging was observed though shaperetention as a whole was good. After the urethane was burned off at1000° C., the residual ceramic reticulum was sintered at 1500° C. forfour hours to bond the fibrils to each other and thereby increase therigidity of the preform.

EXAMPLE F

The preform formed in Example E was subsequently impregnated with analuminum alloy containing 10 weight percent magnesium and 5 weightpercent silicon. More specifically, the preform was set atop a soliddisk of the alloy in an alumina crucible and heated to 900° C. inflowing nitrogen for 15 hours. The aluminum wetted the fibrils andwicked into the preform. After cooling, the sample was sectioned andanalyzed with Electron Probe Micro Analysis (EPMA) and SEM. FIG. 4 showsa cross-section of the thusly impregnated preform and shows that thereticulated structure of the preform is preserved during impregnation.An EPMA elemental map for oxygen in the same area shows that thecircular features 12 in FIG. 4 are high in oxygen content relative tothe Al matrix which is consistent with the presence of Al₂ O₃ thereat.More specifically, EPMA elemental analysis shows these circular features12 to be MgAl₂ O₄ indicating at least some reaction with the Mg present.

While the invention has been disclosed primarily in terms of specificembodiments thereof it is not intended to be limited thereto, but ratheronly to the extent set forth hereafter in the claims which follow.

The embodiments of the invention in which an exclusive property orprivilege is claimed as defined as follows:
 1. A method of making areinforced composite having a desired shape, said composite comprising amatrix phase embedding a reinforcement phase comprising the steps of:a.providing first and second prepolymers adapted to react with each otherso as to foam and form a polymer having a three dimensional, reticulatedstructure comprising a plurality of randomly oriented thread-likeportions interconnected one to the next via a plurality of nodes andtogether defining a plurality of interconnected, interstitial cells; b.mixing a plurality of discrete reinforcing particles with at least oneof said prepolymers prior to mixing the prepolymers together; c. mixingand reacting said prepolymers together in the presence of said particlesin a mold having a mold cavity of a desired shape, and so as to foam thereacting mixture and form said structure in said cavity and toconcentrate said particles at said thread-like portions and nodes to thesubstantial exclusion of said particles from said cells; d. removingsaid polymer while leaving said particles substantially in place; e.bonding said particles together to provide a substantiallyself-supporting reticulated preform conforming to the shape of said moldcavity; and f. impregnating said preform with a matrix material selectedfrom the group consisting of metals and ceramics.
 2. A method accordingto claim 1 wherein said polymer comprises a polyurethane and saidprepolymers comprise a polyol and an isocyanate.
 3. A method accordingto claim 2 wherein said particles are mixed with said polyol.
 4. Amethod according to claim 1 wherein said particles are fibrils.
 5. Amethod according to claim 1 wherein said polymer is a silicone and saidprepolymers are selected from the group consisting of polysiloxanes,silane and silanol.
 6. A method according to claim 1 wherein saidparticles comprise about 3 percent to about 50 percent by volume of theprepolymer with which it is mixed.
 7. A method according to claim 1wherein said particles are selected from the group consisting of Al₂ O₃·SiO₂ (4% SiO₂), Al₂ O₃ ·SiO₂ (53% SiO₂) and SiC.
 8. A method accordingto claim 1 wherein said bonding is effected by sintering.
 9. A methodaccording to claim 1 wherein said particles are coated with SiO₂ andsaid bonding is effected by SiO₂ gluing.